Switch to Python format string.

SoS designed its string interpolation system when it was started before the Python format string was introduced (to Python 3.6). For a while SoS has kept its own string interpolation syntax because SoS supports Python 3.5, a collection of converters that are convenient to use, and its use of ${ } sigil that is much easier to use with scripts in Python and R for which { } is widely used. However, it became clear that our string interpolation system has inferior performance compared to Python's native format string, and it can be really confusing to have two sets of systems in SoS. For these reasons we bumped SoS to require Python 3.6, removed SoS' own interpolation system, introduced a sos_targets data type to keep most of the path conversion features,, and parameter expand and magic %expand to interpolate with { } or alternative sigils. This has resulted in a much cleaner and faster string interpolation system.